4 Pound, peeled, quartered, and cut into 1/4-inch-thick. slices (4 nos.)
6 Large, beaten
In a 10-inch skillet over medium heat, heat the oil and add the onions.
Sprinkle with salt and pepper.
Cook until tender, about 15 minutes.
Pour the onions and oil into a sieve over a bowl.
Return the strained oil to skillet and place over medium heat.
Add the potatoes and sprinkle with salt and pepper.
Cook, tossing occasionally, until lightly golden, about 10 minutes.
Pour the potatoes and oil into the same sieve, reserving the oil (you should have about 1 tablespoon).
In a large bowl, combine the onions, potatoes, and eggs.
Return the oil to the skillet and heat over medium-high heat.
Add the egg mixture and cook until set on the bottom but still runny on the top.
Cover the skillet with a flat lid or a pizza pan.
Quickly invert the skillet, unmolding the cake.
Return to the pan, uncooked side down, and cook until the cake is set and golden brown.
The center should have a little give and the sides should be firm.
Turn the cake onto a platter and let sit for 15 minutes.
Cut into 12 wedges and serve at room temperature.